My system operates at a nominal 13.2V, from an 80Ah LiFePO4 battery pack (made up from 32 cells, connected as an 8P, 4S array). The reduction drive is 2:1 and uses a 3M HTD belt, 15mm wide. The motor is a modified TowerPro 5330, which is no longer made. I haven't rewound it, just reconnected the three windings so they are now in wye, rather than delta (like any three phase motor this gives a reduction factor of 1.73 in Kv, making the motor a 118Kv one instead of a 200Kv one). The motor was also modified to accept three Honeywell Hall sensors internally, bonded into three slots machined into the stator at the correct location.
The controller is a much modified 6 FET electric bike one. I bought some bare boards from a chap in China, modified them to run on low voltage, fitted very low on resistance FETs to reduce losses and programmed it to deliver 30A (battery current) maximum. This controller needs the Hall sensors, just like most non-model aircraft controllers, but does give great performance from a package not much larger than a pack of cigarettes.
There are a lot of reasonably priced outrunner motors around. I also have some big (6.5kW) ones, similar to these: http://www.hobbyking.com/hobbyking/store/__5142__Turnigy_80_100_B_130Kv_Brushless_Outrunner_eq_70_55_.html which are good value at just over $100 each, although of variable quality. If you want more power, then this $300 motor is reputedly good for around 12kW peak: http://www.hobbyking.com/hobbyking/store/__14427__Turnigy_CA120_70_Brushless_Outrunner_100cc_eq_.html
I've found that the smaller motors, around the size of this 2kW, $100 one: http://www.hobbyking.com/hobbyking/store/__17984___Turnigy_RotoMax_1_20_Brushless_Outrunner_Motor.html tend to be better at running efficiently for low power applications. Bear in mind that these motors all have pretty variable assembly quality. The parts are fine, but they are Chinese and need things like bearings replacing with decent ones and the application of conformal coating to the windings to make them reasonably reliable. However, at the price they offer good value for someone prepared to experiment.
Jeremy
--- In electricboats@yahoogroups.com, "fneilss" <fneilss@...> wrote:
>
> Very interesting, Jeremy. What voltage is your system? I assume you are doing some kind of reduction? What sort of controller?
>
> Your system operates over exactly the power range I would like mine to. I had considered outrunner model airplane motors, but couldn't really find one configured the way I wanted - I think I need to look into going the custom winding route you took.
>
> Thanks,
>
> Neil
>
> --- In electricboats@yahoogroups.com, "Jeremy" <jeremy_harris_uk@> wrote:
> >
> > Motor efficiency is determined by a whole bunch of factors, some of which impact on low power performance, some of which impact on performance at any power level and some of which only impact on high power performance.
> >
> > What I did was choose a motor with a high power rating, but with losses that were very low at low power levels, which gave me the efficiency I was looking for.
> >
> > Broadly speaking, brushless motor losses are made up of the following elements:
> >
> > - Frictional loss in bearings and seals, a fairly fixed loss that is there across the power range. Picking a motor with the smallest diameter shaft commensurate with the torque delivery requirement will minimise these losses.
> >
> > - Resistive loss in the motor windings, wiring, controller and battery internal resistance. The lower the motor winding resistance the lower the losses. Higher current rated motors will generally have a lower resistance than those with a lower current rating.
> >
> > - Eddy current loss in motor magnetic material. This loss is proportional to the frequency with which the magnetic field reverses, so proportional to rpm. It tends to be pretty low at normal operating rpm, but can rise to high levels at high rpm with a high pole-pair count motor.
> >
> > - Windage loss. This comes from the aerodynamic drag of the rotating parts and is higher for a large diameter motor and proportional to the square of rpm, so increases rapidly as rpm increases.
> >
> > In my case I chose a motor very carefully to match the needs of my boat system. I used a model aircraft BLDC outrunner motor, rewired from delta to wye to reduce circulating current losses and also to lower Kv. I chose one with a 63mm rotor inside diameter, for low windage losses. It has a small 8mm diameter shaft, which reduces bearing frictional losses. It has a maximum rpm of around 7000 to 8000, but I run it at a maximum of 1500rpm, which both reduces windage losses and also losses from from eddy currents. The maximum current for this motor is around 50 to 60 amps, but I run it at between 10 and 20 amps, which keeps resistive losses low.
> >
> > It's an oddball system, optimised for running a low speed riverboat, so it doesn't scale well, but it does show that if you take the time to choose carefully you can squeeze a fair bit of performance from an electric drive system, accepting the limitations we have with energy storage.
> >
> > Jeremy
> >
>
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